The present invention relates to solar heating systems and, in particular, to a modular passive solar heating system which uses a heat pipe or heat tube absorber.
Passive solar water heating systems are well established in the art and are desirable in many applications because of their characteristic lack of need (or minimum requirements) for electric power, controls and pumps. The so-called water thermosyphon may well be the prevalent passive solar water heating system in terms of worldwide usage. In a typical and simple arrangement, the water thermosyphon uses a non-tracking water absorber for absorbing energy in the form of heat from incident solar radiation, then transfers this thermal energy via an inlet manifold to a water reservoir or tank which is mounted on the absorber.
U.S. Pat. No. 4,217,882, issued Aug. 19, 1980, discloses a two-phase thermosyphon which is an improvement over the relatively heavy freeze-susceptible, conventional water thermosyphons. The '882 patent adapts the well known heat pipe or heat tube concept, originally developed approximately twenty years ago, to a tracking parabolic trough concentrator and also to fixed, cylindrical and compound parabolic concentrators. As applied to such embodiments, the heat pipe comprises an elongated, small diameter tube which at one end partially surrounds or penetrates a water tank. The tube is partially filled with a vaporizable working fluid and is supported at a slight incline relative to the horizontal. Incident solar radiation is focused on the tube by the surrounding concentrator structure heats and evaporates the fluid. The resulting vapor rises to the upper, tank end of the heat tube where, due to thermal coupling, the solar heat is transferred from the tube fluid to the tank fluid, which typically is water. Upon giving up its latent heat of evaporation, the condensed working fluid is returned by gravity to the lower, concentrator end of the tube.
The above-described heat tube imparts several advantages to passive solar water heating systems. The heat pipe has excellent thermal conductance in that it has very high heat transfer capability over even a relatively small temperature gradient. In addition, the evaporation-condensation cycle provides highly anisotropic, essentially one-way heat transfer along the tube. It may help understanding to consider the situation at night or during other periods of low incident solar radiation. At such times, there is little or no evaporation and condensation of the working fluid; the fluid in its liquid state pools at the lower, concentrator end of the heat tube. The resulting discontinuity in the conduction path between the absorber/concentrator and the tank essentially eliminates heat loss via the working fluid. The combined result of the excellent thermal conductance characteristics and the one-way heat transfer characteristics is very efficient heat transfer along the heat pipe into the reservoir with little outward heat loss. Other advantages of adapting heat tubes to passive solar collectors, not exhaustive, include relatively light weight; adaptability to freeze protection, since only the reservoir tank contains water; and a high percentage net usable system energy, since little or no parasitic power consumption is required to operate the system.
One known alternative to the '882 system comprises a flat plate collector array of finned heat tubes which wrap around an integral storage tank. The heat tubes and the tank wall in effect comprise a double-walled heat exchanger so that heat is transferred from the working fluid in each heat tube, through the heat tube and tank walls, and into the storage medium.
Despite the above substantial advantages over prior art passive solar water heating systems, heretofore it has not been possible to fully utilize the potential of the heat tube concept. In particular, the unitary heat collector system exemplified in the '882 patent utilizes a relatively complex collector/concentrator which, like the other prior art systems, is integrated with the tank. It is difficult to disassemble all or part of the '882 system or the alternative system for inspection or repair. It is also difficult to use such systems in a primary applications of passive systems, namely roof top applications on residential dwellings. In short, while incorporation of the heat tube technology into solar heat collector systems represents an advance in passive solar energy technology, the potential of heat tube systems for combined simplicity, durability and ease of maintenance has not been fully realized.